1. Field of the Invention
The present invention relates to an electroluminescent display device, and more particularly, to an organic electroluminescent display device and a method of fabricating the same.
2. Discussion of the Related Art
In general, an organic electroluminescent display device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer, combining the electrons and the holes to generate an exciton, and transiting the exciton from an excited state to a ground state. Since the organic electroluminescent display device does not require an additional light source due to its self-luminescence, the organic electroluminescent display device has a small size and is light weight, as compared to a liquid crystal display device. The organic electroluminescent display device also has low power consumption, high brightness, and short response time. Thus, the organic electroluminescent display device is used in most consumer electronic applications, such as cellular phones, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm PCs. In addition, the organic electroluminescent display device can have reduced manufacturing costs because of its simple manufacturing processes.
Organic electroluminescent display devices may be categorized into passive matrix-type and active matrix-type depending upon the method used to drive the device. Passive matrix-type organic electroluminescent display devices have a simple structure and are fabricated through a simple manufacturing process. However, the passive matrix-type organic electroluminescent display devices have high power consumption, thereby preventing use in large area displays. Furthermore, in passive matrix organic electroluminescent display devices, aperture ratio decreases according to the increasing number of electrical lines. Thus, the passive matrix-type organic electroluminescent display devices are commonly used as small-sized display devices. Active matrix-type organic electroluminescent display (AMOELD) devices are commonly used as large-sized display devices since they have high luminous efficacy, and provide high definition images.
FIG. 1 is a cross sectional view of an active matrix-type organic electroluminescent display (AMOELD) device according to the related art. In FIG. 1, the AMOELD device 10 includes a first substrate 12 and a second substrate 28, which are spaced apart and face each other. A plurality of thin film transistors T and a plurality of first electrodes 16 are formed on an inner surface of the first substrate 12, wherein each of first electrodes 16 are connected to each of thin film transistors T. Organic layers 18 are formed on the first electrodes 16 and the thin film transistors T, and a second electrode 20 is formed on the organic layers 18. The organic layers 18 emit light of three colors: red (R), green (G), and blue (B) within a pixel region P, and are generally formed by patterning an organic material.
A desiccant 22 is formed on an inner surface of the second substrate 28 to remove any external moisture and air that may permeate into a space between the first and second substrates 12 and 28. The inner surface of the second substrate 28 is patterned to form a groove, and the desiccant 22 is disposed within the groove and is fastened with a tape 25.
A sealant 26 is formed between the first and second substrates 12 and 28, and surrounds elements, such as the thin film transistors T, the first electrodes 16, the organic layers 18, and the second electrodes 20. The sealant 26 forms an airtight space to protect the elements from the external moisture and air.
FIG. 2 is a plan view for a pixel of an AMOELD device according to the related art. In FIG. 2, the pixel includes a switching thin film transistor (TFT) TS, a driving thin film transistor (TFT) TD, and a storage capacitor CST. In addition, a gate line 32 and a data line 34 are formed on a substrate 12, and are formed of a transparent material, such as glass and plastic. The gate line 32 and the data line 34 cross each other to define a pixel region P, and a power line 35 is formed parallel to the data line 34.
The switching TFT TS and the driving TFT TD include a gate electrodes 36 and 38, an active layer 40 and 42, a source electrode 46 and 48, and a drain electrode 50 and 52, respectively. The gate electrode 36 of the switching TFT TS is connected to the gate line 32, and the source electrode 46 of the switching TFT TS is connected to the data line 34. The drain electrode 50 of the switching TFT TS is connected to the gate electrode 38 of the driving TFT TD through a first contact hole 54, and the source electrode 48 of the driving TFT TD is connected to the power line 35 through a second contact hole 56. The drain electrode 52 of the driving TFT TD is connected to a first electrode 16 in the pixel region P. A capacitor electrode 15 overlaps the power line 35 to form the storage capacitor CST, and is made of doped polycrystalline silicon and is connected to the drain electrode 50 of the switching TFT TS.
FIG. 3 is a cross sectional view of the AMOELD device along III—III of FIG. 2 according to the related art. In FIG. 3, the driving TFT TD is formed on the substrate 12, and includes the gate electrode 38, the active layer 42, and the source and drain electrodes 48 and 52. An insulating layer 57 covers the driving TFT TD, and the first electrode 16 is formed on the insulating layer 57 to electrically contact the drain electrode 52. An organic layer 18 that emits one color of light is formed on the first electrode 16, and the second electrode 20 is formed on the organic layer 18 over an entire surface of the substrate 12.
FIG. 4 is a cross sectional view of the AMOELD device along IV—IV of FIG. 2 according to the related art. In FIG. 4, the switching TFT TS is formed over the substrate 12, and includes the gate electrode 36, the active layer 40, and the source and drain electrodes 46 and 50. On the other hand, the storage capacitor CST is formed over the substrate 12 and includes the capacitor electrode 15 and the power line 35. The insulating layer 57 covers the switching TFT TS and the storage capacitor CST, and the first electrode (not shown) is formed on the insulating layer 57. Next, the organic layer is formed on the first electrode, and is positioned between adjacent partition walls 70. The organic layer 18 is generally includes an emissive layer, a hole transporting layer, and an electron transporting layer. The emissive layer is disposed between the hole transporting layer and the electron transporting layer. The partition wall 70 corresponds to the data line 34 and the power line 35 to prevent the organic layer from contacting the adjacent pixel region P. The second electrode 20 is formed on the organic emissive layer and on sidewalls of the partition wall 70. An upper part of the partition wall 70 has a width narrower than a lower part of the partition wall 70 to form the second electrode 20 not only on the emissive layers but also on the partition wall 70.
In addition, a yield of the AMOELD device depends on yields of the thin film transistor and the organic layer. The yield of the AMOELD device varies due to impurities in the process of forming the organic layer to a thickness of about 1,000 Å. Accordingly, the yield of the AMOELD is reduced because of the impurities, thereby resulting in a loss of manufacturing costs and source materials for the thin film transistor.
Moreover, the AMOELD device is a bottom emission mode device having stability and degrees of freedom for the manufacturing processes. However, the bottom emission mode device has a reduced aperture ratio. Thus, the bottom emission mode AMOELD has difficulty in being used as a high aperture device. On the other hand, a top emission mode AMOELD has a high aperture ratio, and is easy to manufacture. However, in the top emission mode AMOELD, since a cathode electrode is generally disposed over the organic layer, a choice of material with which to make the cathode electrode is limited. Accordingly, transmittance of light is limited, and a luminous efficacy is reduced. Furthermore, in order to improve light transmittance the passivation layer should be formed as a thin film, whereby the exterior moisture and air is not fully blocked.